I'm thinking now that maybe other races are fighting against the Darkness as well as each other. Bungie did release info that Fallen and Hive are against each other, as are the Cabal and Vex. This lead to the Fallen and Cabal joining forces and the Hive and Vex doing the same(you might wanna double check that just in case lol). Now, I got to thinking because of your theory, if the Hive are Fallen zombies, and if the Fallen are fighting against them, maybe they became zombies because of the Darkness. Now if the Fallen are fighting against the Darkness, and the Cabal joined them, then it would make sense that the Vex are the one's sided with the Darkness. Now, after this is going to be pure theory, no facts to back it up...since we don't know much about the Traveller...who's to say it's not some sort of signal for the Darkness to come and destroy a world in it's prime? And maybe that's where the Vex come in with their creators, yet they've been infected by the Darkness similar to how the Hive were...so yeah...conspiracy lol